erstklassig
first-class
Of the best quality; excellent, outstanding, or superb.
Das Hotel bietet einen erstklassigen Service und luxuriöse Zimmer.
The hotel offers first-class service and luxurious rooms.

first-class
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first-class
Of the best quality; excellent, outstanding, or superb.
Das Hotel bietet einen erstklassigen Service und luxuriöse Zimmer.
The hotel offers first-class service and luxurious rooms.

The word is made up of 'erste' (first) and 'Klasse' (class). It literally means 'of the first class' and describes something of the highest quality, similar to first class on a plane or train.
Für ihre Prüfungsvorbereitung hat sie erstklassige Arbeit geleistet.
She did first-class work in preparing for her exam.
'Erstklassig' (first-class) is often used to praise products, services, or performances. Typical combinations are 'erstklassiger Service' (first-class service), 'erstklassige Qualität' (first-class quality), or 'erstklassiges Essen' (first-class food).
Das Restaurant ist für sein erstklassiges Essen bekannt.
The restaurant is known for its first-class food.
This adjective is a very strong positive evaluation. You often find it in advertising or formal reviews, but also in everyday language to give something special praise.
Die Leistung der Mannschaft im Finale war einfach erstklassig.
The team's performance in the final was simply first-class.
bad
'schlecht' and 'erstklassig' are clear opposites when evaluating quality. 'schlecht' means that something is of low quality or does not work as it should. 'erstklassig' means that something is of the best quality. It sounds stronger than simply 'gut' (good) and fits well with products, service, performance, or organizations. The contrast is: flawed versus excellent.

Der Service im Hotel war erstklassig.
The service at the hotel was first-class.
Die Verbindung im Bus war so schlecht, dass sie niemanden anrufen konnte.
The connection on the bus was so bad that she couldn't call anyone.